Rewrite wxExecute() implementation under Unix.

Rewrite wxExecute() implementation under Unix.

This commit changes wxExecute() to handle SIGCHLD to be notified about the
child process termination instead of detecting when the file descriptor
corresponding to the other end of a pipe opened in the parent process was
closed in the child as this was not reliable and could (and did) result in not
detecting the termination of the child processes that closed all their file
descriptors before exiting.

This commit also removes a lot of platform-specific code duplicating the
generic event loop sources support and reuses it for wxExecute() purposes too.

Final big change is that wxEndProcessData was merged into wxExecuteData and we
don't have two similar but quite different classes any more but just one,
which is used both to pass the information from wxExecute() to wxAppTraits
methods and to store this information until the child termination.

Closes #10258.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74350 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775